The DM42 exists now, so you have two choices. Buy a DM42 now, or wait for the WP43s to exist, which could take many years, if ever.

Or, of course, the third choice; wait for the DM43, which doesn't exist, hasn't ever existed, hasn't been mentioned as ever possibly existing and hasn't been discussed by Walter or Pauli (the W and P in WP43s).

So I go back to my original question ....Why would I buy one instead of the other? (if they both existed today i.e. a DM42 and a DM43)

Take a look at the WP 43S feature list on the project page, it should be more powerful than the DM42, but also may be more complex to use.

I'm not sure the WP 43S will bring anything that will justify buying one for me . I mean I have already a 41CL, a DM42 and a WP 34S, so plenty of RPN power but when I want to do some quick programming or math exploration I just pull the Prime which is way more powerful and easier to program. I still like RPN programming as it was the first programming language I learned but nowadays I'm using it just for programming challenges.
And for calculations on the go a 32SII is perfect.

The obvious answer here, and I'm surprised no one has posted it yet, is get both.

The DM42 is available now and a wonderful machine, with pretty much universal praise. Even if you've never owned an HP-42S, if you know/like RPN, you will find it easy to learn and use, and in time with practice, even the advanced features (e.g. matrix, solver, complex numbers) are pretty easy to learn and use.

The WP43S will contain a similar/familiar but distinctly different keystroke programming language, and similar precision (as the DM42S) but also more features such as additional statistic functions, likely more/better solvers, additional Math functions (many of which I don't understand), roughly similar PC data exchange capabilities, and more. If I were a marketing guy, I might call it a WP34S on steroids, but I'm not, so I won't, but that should complete the idea.

Best GUESS is the WP43S will not be available for 2-3 years, unless some talented developers appear with LOTS of free time very soon, but it will certainly be worth the long wait.

So, yeah, pick the best choice and get both; very unlikely you'll regret it.

The thing I dislike the most about the wp34s is the lack of a built in MISO solver. Sure you can program your own, but it is a very steep learning curve. Even if you know how to build a MISO solver it still takes much longer to input on the WP34s vs the hp35s.

The solver on free42 (hp42s) is a thing of beauty.

I'm under the understanding the main reason there isn't a better solver on the WP34s is due to memory limitations. But honestly I'd delete most of the other built in functions in the flash memory and replace it with a general miso solver if you gave me the code. I'm a petroleum engineer so most of those amazing features you have included I have never used and will never use. I have programmed my own functions into the calculator but I use miso solvers so often, that I pick up my other calculator more often. I need the flexibility of a dedicated MISO solver in my work flow.

If the WP43s is the same as the WP34s but with a solver as good as the hp42s, I'd buy it on day one.

I'm still contemplating buying a DM42s. The main reason for contemplating is because I just bought a WP34s and haven't got enough out of it yet to justify how much I spent too aquire it. So I don't want to go out and buy a new calculator so fast.

The thing I dislike the most about the wp34s is the lack of a built in MISO solver. Sure you can program your own, but it is a very steep learning curve. Even if you know how to build a MISO solver it still takes much longer to input on the WP34s vs the hp35s.

The solver on free42 (hp42s) is a thing of beauty.

I'm under the understanding the main reason there isn't a better solver on the WP34s is due to memory limitations. But honestly I'd delete most of the other built in functions in the flash memory and replace it with a general miso solver if you gave me the code. I'm a petroleum engineer so most of those amazing features you have included I have never used and will never use. I have programmed my own functions into the calculator but I use miso solvers so often, that I pick up my other calculator more often. I need the flexibility of a dedicated MISO solver in my work flow.

If the WP43s is the same as the WP34s but with a solver as good as the hp42s, I'd buy it on day one.

I'm still contemplating buying a DM42s. The main reason for contemplating is because I just bought a WP34s and haven't got enough out of it yet to justify how much I spent too aquire it. So I don't want to go out and buy a new calculator so fast.

I'm a working engineer myself, and the solver on the HP42 has saved me so many times that it's become a crutch. When my original died, I tried the solver on the 35S, but the single-letter variable names was a show stopper for me. I'd spent so many years solving for Ft of Head (FtHead), or Disentegrations per minute (DPM), or seconds (Sec), or whatever, that trying to use a single letter just made my brain hurt. Luckily, my 35S died just as the beta program started, so I was able to get in at the beginning.

The other reason I pulled the trigger on the DM42 is that it uses Free42 as the core. Free42 has been tested in so many edge cases that I feel completely comfortable using it for any application, up to life-safety calculations. When I look at the errata for the HP35S http://www.hpmuseum.org/cgi-sys/cgiwrap ... i?read=735, especially number 14, I don't get that confidence.